Peripheral blood progenitor cells (PBPC) are the most popular source of hematopoetic stem cells for allogeneic transplantation because of technical ease of collection and faster engraftment. Traditionally, granulocyte-colony stimulating factor (G-CSF) has been used to procure the peripheral blood stem cell graft. Although regimens using G-CSF usually succeed in collecting adequate numbers of PBPC from healthy donors, 5%-10% of the donors will mobilize stem cells poorly and may require multiple large volume apheresis or bone marrow harvesting. AMD3100 reversibly inhibits CXC- chemokine receptor 4 (CXCR4) binding to stromal cell derived factor (SDF) - 1 and was recently discovered to be an effective agent to mobilize cluster of differentiation 34 (CD34)+ cells into the peripheral blood. In normal volunteers, administering AMD3100 after 4-5 days of G-CSF resulted in a 3-3.5 fold increase in circulating CD34 cells compared to G-CSF alone. Recent data has suggested that the combination of G-CSF and AMD3100 is superior to G-CSF alone for mobilizing hematopoietic progenitor cells in heavily pretreated patients with multiple myeloma or non-Hodgkin's lymphoma undergoing autologous hematopoietic transplantation. Combining AMD3100 with G-CSF could be an effective strategy to improve the yield of PBPC collected from allogeneic donors who mobilize poorly with G-CSF alone. However, the biological impact of AMD3100 in this context on T cells and other cellular populations contained within the allograft that mediate graft versus host disease (GVHD) and graft-versus-leukemia (GVL) effects are unknown.